The present invention relates to a microwave bandpass filter provided with a resonator having line patterns formed on a substrate.
Conventionally, microwave filters have been constructed as shown in FIGS. 1 and 2.
FIG. 1 shows a microwave filter formed with line patterns including an input line 11, a pair of half-wavelength resonators 12, and an output line 13. A geometrical feature of the microwave filter of this type is that the half-wavelength resonators 12 are disposed between the input and output lines 11 and 13 and extend parallel to one another. A microwave signal applied through the input line 11 causes the pair of half-wavelength resonators 12 to resonate so as to produce electric power at a desired frequency upon the output line 13.
In the microwave filter of this type, however, there are disadvantages that a relatively large area is required for the respective line patterns of the input line 11, the half-wavelength resonators 12, and the output line 13 on the substrate, and a large number of stages are required in order to make sufficiently wide the bandwidth of the microwave filter, resulting in a large loss.
FIG. 2 shows another microwave filter provided with an input line 21, a pair of half-wavelength annular resonators 22, and an output line 23. A microwave filter of this type is disclosed, for example, in Japanese Patent Publication No. 7721/1980. In this filter, the pair of half-wavelength resonators 22 disposed between the input and output lines 21 and 23 are geometrically configured such that a pair of annular strip lines each having a gap G are arranged so as to form respective gaps G in opposition to each other. The function of the microwave filter of FIG. 2 is the same as that of FIG. 1.
In the microwave filter of the type shown in FIG. 2, although it is possible to reduce the area occupied by the line patterns, there are still disadvantages that it is difficult to make the bandwidth of the filter sufficiently wide and that there is a considerable amount of loss (fractional bandwidth of several percent).